Investigation of the AFm-Cl system: Fe-to-Al solid solution, thermal behavior and carbonation

Abstract

Due to increasing interest in reducing CO₂ emissions, new hydraulic binders are emerging, many of which contain significant amounts of iron and aluminum oxides. The reactivity of such binders can be activated using CaCl₂. It thus appears essential to investigate mixed Al/Fe-Cl hydrates to better characterize the hydration products and hydration processes of these new hydraulic binders. AFm-Cl phases with varying proportions of Al³⁺ and Fe³⁺ trivalent cations were synthesized and characterized. Carbonation of AFm-Cl samples is unavoidable at ambient atmosphere and leads to phases with Ca₂Al_1-xFe_x(OH)₆·Cl_0.7(CO₃)_0.15·2H₂O general composition (samples were synthesized for x = 0, 0.17, 0.33, 0.50, 0.67, 0.83 and 1). A complete solid solution between the two Ca₂Al(OH)₆·Cl_0.7(CO₃)_0.15·2H₂O and Ca₂Fe(OH)₆·Cl_0.7(CO₃)_0.15·2H₂O endmembers was demonstrated. Two hydration states were identified with characteristic interlayer distances of about 7.8 Å and 6.8 Å for, respectively, the Ca₂Al_1-xFe_x(OH)₆·Cl_0.7(CO₃)_0.15·2H₂O hydrate and the Ca₂Al_1-xFe_x(OH)₆·Cl_0.7(CO₃)_0.15 dehydrated phase. Thermal studies indicated a similar succession of thermal events for Ca₂Al_1-xFe_x(OH)₆·Cl_0.7(CO₃)_0.15·2H₂O regardless of the nature of the trivalent cation; the presence of iron however decreasing the degradation temperatures. Finally, the Ca₂Al_1-xFe_x(OH)₆·Cl_0.7(CO₃)_0.15·2H₂O phase was investigated in the presence of carbonate anions, showing a progressive exchange of chloride for carbonate within the interlayer to form Ca₂Al_1-xFe_x(OH)₆·(CO₃)_0.5·3H₂O before degrading into calcite.